1. Field of the Invention
The present invention relates to a device for determining the abnormal degree of deterioration of a catalyst.
2. Description of the Related Art
A catalyst is usually arranged in an exhaust passage to purify the exhaust gas. Once the catalyst deteriorates excessively, the catalyst cannot purify the exhaust gas sufficiently. Accordingly, it is necessary to determine the abnormal degree of deterioration of the catalyst, to inform the driver about this, and to urge the driver to exchange the catalyst for a new one.
The exchange is costly and requires much time. Accordingly, it is necessary to determine the abnormal degree of deterioration of a catalyst precisely. In a usual device for determining the abnormal degree of deterioration of a catalyst, the current purification ability of a catalyst is detected after the catalyst has been activated and, thereafter, the current purification ability is compared with the threshold which corresponds to the abnormal degree of deterioration thereof. In a catalyst, only a purification ability before the catalyst has been activated can drop excessively. In this case, the usual device determines that the degree of deterioration of the catalyst is normal. Therefore, the catalyst is not exchanged and thus the exhaust gas is not completely purified before it has been activated.
To solve this problem, Japanese Unexamined Patent Publication No. 5-248227 discloses a device for determining the abnormal degree of deterioration of a catalyst. The device corrects the threshold which corresponds to the abnormal degree of deterioration of a catalyst, according to the degree of activation thereof, i.e., the temperature thereof, detects a current purification ability thereof before and after the catalyst has been activated, compares the current purification ability with the corrected threshold, and determines if the degree of deterioration thereof is abnormal.
However, a purification ability of a catalyst before the catalyst has been activated is particularly unstable, because it varies largely according to not only the temperature thereof but also the harmful materials content of the exhaust gas and the like. Accordingly, if the device merely compares the current purification ability with the corrected threshold and determines the degree of deterioration of the catalyst, the device can mistake the determination.